1. Field of the Invention
This invention relates to a developing apparatus and a process cartridge, which are used in an electrophotographic image forming apparatus, and to an electrophotographic image forming apparatus using those apparatuses.
Herein, the electrophotographic image forming apparatus means an apparatus for forming images on a recording medium using an electrophotographic image forming process. For example, the electrophotographic image forming apparatus includes, e.g., an electrophotographic photocopier, an electrophotographic printer (e.g., LED printer, laser beam printer), an electrophotographic facsimile machine, and an electrophotographic word processor.
The process cartridge discussed herein refers to an apparatus made as a cartridge integrated with at least a developing apparatus and an electrophotographic photosensitive drum to be detachably mountable to an image forming apparatus body (the main assembly of the image forming apparatus). The developing apparatus discussed herein refers to an apparatus made as a cartridge integrated with a toner container and a developing means to be detachably mounted to an image forming apparatus body.
2. Description of Related Art
A process cartridge system has been used conventionally in which an electrophotographic photosensitive member and a developing means are integrated into a cartridge to be detachably mounted to an image forming apparatus body.
With such a cartridge method, the controllability is further improved to allow users themselves to easily do maintenance work on a processing means. This cartridge method is consequently used widely for image forming apparatus.
A cartridge structure is also realized in which processing means are divided into ones having a longer duration and ones having a shorter duration and are made into cartridges to be used according to the length of the duration of a main processing means. For example, a developing cartridge integrated with a toner container and a developing means, and a drum cartridge integrating an electrophotographic photosensitive drum, a charging means, and a cleaning means are used.
With such a cartridge, as a conventional developing method, a so-called magnetic brush developing method has been known. In this method, first, a two-component developing agent made of a non-magnetic toner and a magnetic carrier is used. A magnetic brush is formed on a surface of a developing agent carrier (hereinafter referred to as “developing sleeve”) in which a magnet is disposed inside. This magnetic brush is made to rub on or is placed adjacently to the photosensitive drum facing to the developing sleeve with a small developing gap. In continuous application of an electric field between the developing sleeve and the photosensitive drum, development is performed by transferring and reverse transferring toner particles between the developing sleeve and the photosensitive drum repeatedly.
It is to be noted that a plate shaped metal member made of a non-magnetic material or a magnetic material is used as a layer thickness limiting member (hereinafter referred to as “blade”) for limiting a layer thickness of a magnetic brush formed of the two-component developing agent. Adjustment of a gap determining the layer thickness of the magnetic brush, or namely the gap between the developing sleeve and the layer thickness limiting member (hereinafter also referred to as “SB gap”) is easy where the blade's surface facing the developing sleeve orients toward the center of the developing sleeve. Therefore, such a structure is conventionally used widely and described in, e.g., Japanese Patent Application Publications (Laid Open) No. 2002-214,919, No. 2002-244,425, No. Heisei 02-173,681, and No. Hei 05-027,574.
However, the above structure has a very small SB gap, 200 to 500 microns. Therefore, during transportation of the products, the developing sleeve may be bent to shift the SB gap. Such a shift in the SB gap may bring inferior quality of images.